1. Field of the Invention
This invention relates to the etching of substrates essentially consisting of elementary silicon, or having a surface layer essentially consisting of elementary silicon and which are hereinafter called silicon-substrates, or Si-substrates.
2. Description of the Prior Art
Si-substrates are needed for many purposes of commercial semi-conductor technology and it often becomes necessary that they be etched selectively during their processing. In this connection, etching must be understood quite generally as the removal of predetermined partial areas of the surface of a Si-substrate to a predetermined depth. In particular, etching here must be understood as the chemical treatment of Si-substrates having been provided with etching masks.
Because of the amphoteric properties of silicon, either acid etching means, for instance HF in aqueous solution, or alkaline etching means, for instance anorganic bases, such as KOH, or organic bases such as ethylen-diamine/catechol solutions (also called EDP solutions) may be used.
For the covering of the surface areas of the Si-substrate which are not to be etched, protective layers, or etching masks are used which are either applied to the substrates, as, for instance, masking lacquer or vacuum-vaporized metal coatings, or by local chemical transformation, for instance by oxidation or nitrogenization of the Si-surface. Thereby, the openings in the mask may be created either by selective application of the masking compound onto the areas to be masked, or by selective removal of the masking layer from the areas to be etched at the Si-substrate surface.
The masks for acid, especially for HF-containing etchants usually are formed by masking lacquers, such as photo lacquers on the basis of organic polymers with more or less pronounced acid-resistance but with a low alkali-resistance, especially whenever relatively shallow etchings depths are to be produced; for alkaline etchants, such as concentrated KOH- and EDP- solutions, epitaxially grown silicon-oxide- or silicon-nitride- layers respectively, or vacuum-vaporized metal coatings (for instance, Cr-Au) are utilized for the masking layer.
This means that the utilization of masking lacquers is limited to acid etchants and relatively shallow etching depths. The metal masks which are resistant to alkaline etchants can only be utilized with limitations, since the introduction of foreign metals is usually undesirable in semi-conductor technology, and particularly if processing involving increased temperatures should have to follow the etching process.
Masking layers consisting of epitaxially grown silicon-nitride have the disadvantage that they are relatively hard to etch, particularly if exact etching structures are to be formed; nitride-forming treatment also is expensive. Masking layers consisting of epitaxially grown silicon oxide are attacked by hot alkaline etchants; since the attack of alkaline etchants on SiO.sub.2 usually is considerably slower (for instance with hot KOH 1:2000, relative to (100) silicon), than on the Si-substrate, it would be possible to balance the lacking resistance of the protective layer by a correspondingly thick SiO.sub.2 layer at the surface of the Si-substrate. This procedure, however, has practical limitations, since the epitaxial growth of a SiO.sub.2 layer on the Si-substrate is relatively slow. To give an example, the etching of Si-substrates under the above described conditions (hot KOH, Si(100)) to etching depths of 1000 .mu.m would require protective layers of epitaxially grown SiO.sub.2 with a thickness of at least about 6 .mu.m. In order to form protective layers of this type, the Si-substrate would have to be heated to 1100.degree.-1200.degree. C. in the oxidizing atmosphere for several hundreds of hours.
It is a known fact (IEEE Transactions on Electron Devices, 25, 1978, pages 1178-1184, and pages 1185-1193) that the etching speed (etching rate, e.g. in .mu.m etching depth per time unit at a given temperature) depends on the crystallographic structure of the etched Si-layer and that the etching rate decreases in the sequence of (100)&gt;(110)&gt;(111). The above cited example, however, is already based on an optimum condition, so that the so-called anisotropic etching of silicon does not present a method which solves the problem of insufficiently resistent protective layers.
There have been attempts (see literature quoted above) to avoid the relative low resistance of SiO.sub.2 -protective layers or masks, respectively, by the use of etchants which attack the SiO.sub.2 -coat to a lesser degree than alkaline metal hydroxide, for instance, on the basis of ethyl-diamine and catechol (also referred to as EDP-solutions), or in some cases by hydrazine, in an aqueous solution. It is a disadvantage in this connection that the silicon-etching rate depends on the doping level of the Si-substrate, and the etching process can take place only in a non-oxidizing atmosphere.
Thus, in summarizing the state of the art of the etching of Si-substrates, it may be said that the problem of the protective coating, especially in the masking area, has not been satisfactorily solved.